1. Field of the Invention
The invention relates to monitoring apparatuses and methods for monitoring the operation state of an internal combustion engine, and in particular relates to such apparatuses and methods in which the total intake air amount is monitored using an atmospheric pressure calculated though a particular learning process.
2. Description of the Related Art
In recent years, various measures have been taken to reduce the environmental stress due to emissions of motor vehicles. As one of such measures environment-oriented internal combustion engines have been developed which reduce the emissions by optimizing the throttle opening degree, the ignition timing, the fuel injection timing, the fuel injection amount, the operation timing of intake and exhaust valves, and so on, under various conditions.
Such optimization control by environment-oriented internal combustion engines uses a particular control logic when the internal combustion engine has been started at a lower temperature. Because an internal combustion engine runs by combusting fuel, the temperature of the internal combustion engine becomes high as it continues to run. Therefore, typically, internal combustion engines are designed such that an optimum combustion state is accomplished at a certain high temperature. Meanwhile, in the case where the internal combustion engine has been started after it was stopped for a long period of time, the temperature of the internal combustion engine remains low for a while after the engine start and in such a state, a desired combustion state may not be accomplished. To cope with this, the aforementioned cold-engine control logic is used to optimize fuel combustion in a cold condition immediately after the engine start.
The exhaust substances that are emitted from an internal combustion engine in a cold condition immediately after the engine start are called “cold-start emissions”, and the engine control for reducing such cold-start emissions is called “CSS (Cold Start Strategy) control”.
Some of the motor vehicles incorporating an internal combustion engine enabling the CSS control are provided with a monitoring apparatus for monitoring the state of the CSS control, and such a monitoring apparatus is called “CSS monitor” and used to monitor whether the CSS control is being or has been properly executed. More specifically, the CSS monitor ascertains the presence of an abnormality in the operation state of the internal combustion engine based on, for example, the amount of intake air to the internal combustion engine.
Because oxygen is used to combust fuel in an internal combustion engine, the necessary amount of air changes according to the air density. That is, the optimum amount of air changes according to the atmospheric pressure, and therefore a reference value used by the CSS monitor needs to be changed according to the atmospheric pressure.
In order to change such a characteristic value according to the atmospheric pressure, typically, a learning process based on a certain record is employed. For example, Japanese Patent Application Publication No. 11-294233 (JP-A-11-294233) recites an idling speed control apparatus for an internal combustion engine. This idling speed control apparatus controls the idling speed of the internal combustion engine to a target speed by estimating the intake air amount accurately without using any atmospheric pressure sensor even when the vehicle has moved from a low altitude to a high altitude and thereby minimizes the deviation of the actual air-fuel ratio in exhaust gas from the target air-fuel ratio and thus an increase in the exhaust emissions. The idling speed control apparatus has an ISC airflow rate learning value updating portion that updates a learning value used to correct the flow rate of air flowing in a bypass passage, which rate depends on the degree of opening of a flow rate control valve, in accordance with the airflow rate measured by an airflow meter.
In order to achieve a sufficiently high calculation accuracy in the aforementioned learning process, the degree of learning (e.g., the number of times the learning value has been updated) is required to be sufficiently high. That is, a sufficient calculation accuracy can not be achieved as long as the learning degree is low.
Therefore, the atmospheric pressure can not be accurately detected after the learning value has been reset (zeroed) as a result of excessive power discharge from a battery (so-called “dead battery”) or as a result of a control unit being replaced with a new one. In such a case, therefore, there is a possibility that the operation state of the internal combustion engine is erroneously determined as being abnormal despite that the internal combustion engine is operating normally. Note that other related technologies are described in Japanese Patent Application Publications No. 07-247888 (JP-A-07-24788), No. 2005-325794 (JP-A-2005-325794), and No. 2004-052599 (JP-A-2004-052599).